With the advance of large-sizing of injection molding machines, a powerful die fastening force is required for a die fastening device, while an increase in operational speed is required for satisfying the demand for enhancing the productivity. A need of reducing the installation space of the injection molding machine is also important since the machine has a large size. Accordingly, as the first background area, there has been increasing prosperous use of a method which utilizes a horizontal injection molding machine in which the opening and closing of the dies are effected by extending and retracting a small diameter, long cylinder assembly at a high speed but with a low drive force, and the dies are powerfully fastened by a large diameter, short cylinder assembly before the die fastening reaction engagement is locked in a die closing condition in which the dies are close together (for example, refer to Japanese Laid-Open Pat. No. 1-232,005). Further, in the production of a large molded article, a molding method using a vertical press device has been proposed (refer to Japanese Patent Publication 3-6880 and Japanese Laid-Open Pat. No. 2-172,711).
Next, as the second background area, a compression molding method in which pressing is carried out during or after injection, and an injection and compression molding method in which pressing is initiated after or just before completion of injection in order to eliminate distortion and to enhance the quality of the outer surface of a molded article, have been proposed, for example, as disclosed in Japanese Patent Laid-Open No. 62-268,620 .
Further, as the third background area, an injection molding machine has injected molten resin at a high pressure, in a range of several hundred kilograms/square centimeter to several thousand kilograms/square centimeter, into a mold cavity defined by a movable die and a stationary die, from a heated cylinder by advancing a screw when the movable die and the stationary die are closed together, i.e., abutting against each other. Recently, the injection volume of molten resin has been changed in accordance with a charge volume within the mold cavity, and has been regulated by the advance speed of the screw. Further, an injection and compression molding method as disclosed in Japanese Laid-Open Pat. No. 62-268,620 has been used in order to enhance the quality of a molded article.
Each of these background areas has problems which need to be solved. In the first background area as disclosed in the Japanese Laid-Open Pat. No. 1-232,005, where a die fastening device is provided in the injection molding machine, the production of a large-size molded article requires an injection pressure of several thousand kilograms/square centimeter in order to feed molten resin into every nook and corner of the mold cavity, and accordingly, the die fastening device which has to receive a reaction force becomes very large.
In the first background area as disclosed in Japanese Patent Publication No. 3-6880 and Japanese Laid-Open Pat. No. 2-172,711, problems similar to that with the system of Japanese Laid-Open Pat. No. 1-232,005 are encountered. Further, a compression molding apparatus using a vertical press device which is operated at a low pressure during injection has been proposed. However, a large floor area is required for arranging the press device and the injection device, and the overall height of the apparatus becomes greater. Further, unlike a horizontal type, the vertical press device cannot use a crane or the like for replacing dies, so that a delivery device is complicated, and many manhours are required. Also, the delivery of a molded article cannot be made by using a conventional device, and the floor area and the operating space required for a robot become larger. Moreover, the delivery of a molded article requires the gripping of opposite ends of the article, and accordingly, a robot cannot be readily used therefor.
A problem in the second background area occurs with the molding of a large molded article, in that an injection pressure of several thousand kilograms/square centimeter is required in order to feed resin into every nook and corner of the mold cavity, since the die fastening device is used in the injection molding machine disclosed in the Japanese Laid-Open Pat. No. 1-232,005. Accordingly, the size of the die fastening device which receives a reaction force becomes extremely large. Further, the control of the wall thickness of a molded article in the pressing direction in the compression molding method is difficult, with the result that the wall thickness is uneven. Also cold marks and flow marks are likely to occur so that the quality of the outer surface of a molded article is inferior. Further, the control of the speed of the flow front is difficult. Also, as the degree of opening of the dies in the injection and compression molding method is small, a pressure equal to that of injection molding is required, with the result that the die fastening device becomes extremely large. Further, joint molding and deep-shape molding cannot be readily made. Further, the control of speed of the flow front becomes concentric, similar to that of the injection molding.
In the molding method disclosed in Japanese Laid-Open Pat. No. 62-268620, the compression stroke is too short to sufficiently feed resin, with the result that the molding of a complicated shape molded article or a large size molded article is very difficult. Further, since resin has to be injected in an amount which is in excess of the desired weight of the molded article, a pressure releasing part is required for the resin, with the result that the shape of dies is complicated.
As a problem of the third background area, the injection molding machine measures the position of a screw or a injection cylinder assembly in order to obtain an injection timing or a variable injection volume in accordance with a variation in the desired charge volume, but the desired injection volume for an article is not obtained because a hold pressure is applied, after completion of an injection process, to a molded article which is then supplemented with additional molten resin during the cooling of the molded article in order to prevent the occurrence of a shrinkage cavity or the like. Further, in the compression molding machine, the accuracy of the mass of molten resin corresponding to the mass of a desired molded article is generally inferior, and the control of the wall thickness of the molded article in the pressing direction is difficult such that the wall thickness becomes uneven.
On the other hand, in the process disclosed in Japanese Laid-Open Pat. No. 62-268,620, a safety valve (pressure releasing part) is provided in the dies in order to discharge excess molten resin when the compression and drawing are made, as the mass of the molten resin injected into the mold cavity is in excess of the mass of the molded article to be produced, in order to reduce the unevenness of the wall thickness and to enhance the quality of the molded article. Accordingly, the dies are complicated, and after completion of the mold article, the pressure releasing part has to be removed, resulting in useless manhours and wasted resin.